We created a cache location in UTM zone 18N coordinates (the datum is WGS 84). Our GPS machine provided coordinates in UTM, but to make them integrate with the image, they need to be in State Plane, which is what our Orthophoto is referenced in. Here are the UTM Coordinates (zone 18N)
Easting 0702893 Northing 4695906
In this exercise, we will map this point over an orthophoto of the UMASS campus.
You can get this photo off the MassGIS website, but to save time, we have uploaded it onto our server. You can get it here. This particular photo is a 1:5,000 Black & White Digital Ortho Image. If you were to download the image from MassGIS, you would have to find the reference sheet that shows which number orthophoto covers the UMass campus. Again, we've simplified this for the purposes of the assignment, as you can see from the image below:
Figure 1: OQ Ids for Amherst
Open QGIS and add the orthophoto quad. Define the projection: State Plane, NAD83, Mass Mainland.
Now we need to be careful with the projections or the data won't overlay properly. QGIS can handle "on the fly projection" for VECTOR layers. But here, we are combining raster (the orthophoto) and vector (the GPS point layer) into the same project. After repeated tests, we confirmed that if we bring the GPS layer into QGIS in UTM coordinates, the "on the fly projection " will not only fail, but the program will become very unstable and eventually crash. QGIS does not provide the functionality of projecting interactively vectors or rasters. (There are open source libraries that deal with this problem (GDAL/OGR): they are tools to translate among raster and vector formats, but we won't introduce them in this course).
The easiest solution will be to convert the UTM coordinates into State Plane, so they will match with the raster. We found some applets in the web that will help you with this translation in two steps: First, you will convert the UTM coordinates to Geographical (i.e., latitude/longitude) at this website:
http://jeeep.com/details/coord/
(note: the above site seems to be a little unpredictable. But there are lots of other UTM to Geographic coordinate converters on the web (do a Google search on "UTM to Geographic conversion and you'll see what we mean). Another site we found is at the Canadian Spatial Reference System at http://www.geod.nrcan.gc.ca/apps/gsrug/index_e.php.
But for now, we'll continue with screenshots of the jeeep.com site. Introduce the UTM coordinates listed for the cache above and click "submit". You should see something similar to the image below:
Figure 2: UTM to Geographical
But we still need the coordinates in the State Plane system, not simply geographical. To do this conversion, you can go to http://www.ngs.noaa.gov/cgi-bin/spc_getpc.prl and enter the Geographical coordinates you just obtained to translate them into the State Plane coordinates that our orthophoto is in. You will need to rewrite them as an integer string (i.e. N422320.2 and W0723206.5):
Figure 3: Geographical to St.PL.
These are the GPS coordinates for the cache location in Mass State Plane system:
Figure 4: State Plane Coordinates